The present invention relates to a thermal-printing device which performs thermal printing by selectively flowing a current to a plurality of heat generating elements.
Thermal-printing devices are generally divided into two types; those which print on a heat sensitive paper sheet and those which print on a paper sheet through an ink ribbon coated with a thermally melting ink. A thermal-printing device of either type can print clearer data than a line printer or the like. Furthermore, with recent developments in semiconductor techniques, heat generating elements can be formed at a finer pitch, which results in printing at a resolution as high as 8 dots/mm. Therefore, a thermal-printing device can print fine images including characters or halftone portions with high quality reproduction characteristics. In view of such advantages, thermal-printing devices are being used not only in the field of OA equipment such as in facsimile system but also in the field of bar code printing.
A bar code is used to express a number having a plurality of digits in a form such that each digit has 7 modules in accordance with a relevant standard such as the Japanese Article Numbering system (JAN), the Universal Product Code (UPC), and the European Article Numbering system (EAN). The 7 modules of a digit "5" including an odd parity, for example, are expressed by "0110001" (where "1" represents black). Each module corresponds to a width of 0.33 mm if the magnification factor is 1. A standard version is formed of 13 digits for each such number, each digit being expressed by 7 modules. The standard version is read by a laser scanner or the like and is registered in a register.
FIG. 1 show a conventional thermal-printing device. The thermal-printing device has 256 resistors or heat generating elements R1 to R256, and 256 diodes D1 to D256 each having its anode connected to the one terminal of a corresponding resistor. In the device shown in FIG. 1, these resistors R1 to R256 and diodes D1 to D256 are divided into eight groups. Thus, each group includes 32 resistors and 32 diodes. The other terminal of each of the resistors R1 to R32 and R225 to 256 in the first and eighth groups is connected to a common node and thence to a power supply terminal VC through a pnp transistor TR1. The other terminal of each of the resistors R33 to R64 and R193 to R224 of the second and seventh groups is connected to a common node and thence to the power supply terminal VC through a pnp transistor TR2. Similarly, the other terminal of each of the resistors R65 to R96 and R161 to R192 of the third and sixth groups, and the other terminal of each of the resistors R97 to R128 and R129 to R160 of the fourth and fifth groups are connected to corresponding common nodes and thence to the power supply terminal VC through respective pnp transistors TR3 and TR4.
The thermal-printing device shown in FIG. 1 further has a data generator 1 for generating timing signals and printing data, a common electrode selection circuit 2 which controls the conduction state of the transistors TR1 to TR4 in response to the timing signals from the data generator 1, and latch circuits 3 and 4 which latch first and second printing data, respectively, from the data generato 1. The latch circuit 3 has first to 32nd output terminals which are respectively connected to the cathodes of the first to 32nd diodes of each of the first to fourth groups of diodes. The latch circuit 4 has first to 32nd output terminals which are respectively connected to the cathodes of the first to 32nd diodes of each of the fifth to eighth groups of diodes.
The data generator 1 includes a data processor which generates a timing signal at a predetermined interval and generates the first and second printing data stored in a memory. In response to the timing signal from the data generator 1, the selection circuit 2 supplies the selection signals shown in FIGS. 2(A) to 2(D) to the transistors TR1 to TR4 so as to sequentially turn them on. When a low-level signal is supplied to the base of the transistor TR1, for example, the transistor TR1 is turned on. Then, a power supply voltage is supplied to the resistors R1 to R32 and R225 to R256 of the first and eighth groups through the transistor TR1. As shown in FIG. 2(E), a current flows through selected ones of the resistors R1 to R32 and R225 to R256 corresponding to those of the diodes D1 to D32 and D225 to D256 which are selected in accordance with the printing data stored in the latch circuits 3 and 4, as shown in FIG. 2(E). The selected resistors are heated. A similar operation is repeated and the data stored in the latch circuits 3 and 4 is sequentially printed on a recording paper sheet. In this case, in synchronism with the printing operation, the recording paper sheet is fed in a predetermined direction to make a label on which the item name, price, weight and the like are printed, as shown in FIG. 3.
When a bar code is printed, if one heat generating element is broken, the code "0110001" representing a number "5" is erroneously printed as a different code "0100001" or "0110000". However, when a printed bar code is read, either a check digit calculation or a parity check is performed. Therefore, a bar code which is erroneously printed almost never leads to a reading error.
However, since a bar code is printed and read even if a single heat generating element has broken down, the operator cannot easily detect a broken down element. Checking for a broken down element through printed bar codes is time- and labor-consuming.